Introduction
In an effort to increase food security, seaweed has emerged as a sustainable food option with multiple health benefits due to its nutritional value . Seaweed can be cultivated either on horizontal/vertical line systems or nets. The latter can yield higher biomass for S. latissima . Offshore seaweed cultivation offers more space compared to nearshore, where activities such as shipping, tourism, and fishing take place. However, moving from nearshore to the offshore high-energy environment increases the hydrodynamic force acting on the biomass, which raises the mooring load and design requirements . This highlights the importance of accurately predicting the hydrodynamic loading on an offshore seaweed cultivation system. To that end, a physical model test campaign has been conducted in Deltares' Atlantic Basin, a wave-current basin, to quantify the hydrodynamic load on a seaweed cultivation net subjected to wave and current loading. Additionally, the impact of biomass on the hydrodynamic loading was investigated. The test setup and measured forces will be made available as open-access datasets.
Materials and methods
Surrogate seaweed made of polyvinyl chloride (PVC) were used for the experiment instead of live seaweeds. The material was chosen based on its similarity to S. latissima in terms of density and flexural rigidity, the latter obtained from a cantilever test . Two identical 4.0 m x 0.8 m field scale nets were used, one kept clean and the other with those surrogates attached. The net was connected to four load cells, one installed at each corner, measuring the load along three axes (Figure 1). For the steady flow tests, four different current speeds were imposed: 0.10, 0.20, 0.30, and 0.38 m/s. For the oscillatory flow tests, twelve regular wave conditions were tested. Each test was repeated for three different orientations with respect to the incoming current/waves: 0°, 45°, and 90°. />
Figure 1. Seaweed net test setup in 90 degree orientation (wave/current perpendicular to the net)
Figure 1. Seaweed net test setup in 90 degree orientation (wave/current perpendicular to the net)
Results and discussion
For the steady flow tests in 90° orientation (current perpendicular to the net), the presence of surrogates increased the measured forces by up to a factor of 10, depending on the surrogate configuration. When the surrogates were positioned on the downstream side of the net, they were lifted by the current, thereby creating openings in the net and reducing form drag. In this configuration, the main contribution to the total force came from skin friction drag induced by shear forces. In contrast, placing the surrogates on the upstream side created full blockage of the net, which induced a much larger force, coming from normal pressure forces and shear forces. At a current speed of 0.38 m/s, varying the surrogate configuration produced a wide range of measured forces (81.2 N to 460.2 N). In terms of net orientation, a 3-fold load reduction was observed when the clean net was aligned with the current direction. For the net with surrogates, the load reduction was up to 11-fold. The same trend applied to the regular wave tests, where the maximum forces were measured when the net was oriented perpendicular to the wave direction. The presence of biomass increased the force by a factor of 1.6 for the lowest Keulegan-Carpenter (KC) number and by a factor of 2.6 for the highest KC number. Furthermore, second- and third-order harmonics were observed in the measured wave amplitudes and were also visible in the measured forces.
Conclusion
An experimental campaign has been conducted in a wave-current basin to measure the hydrodynamic forces on a seaweed cultivation net. The experiment has shown that net orientation and the presence of biomass can lead to significant variations in the measured hydrodynamic forces. Both factors need to be accounted for in the design of offshore seaweed farms. This study provides open-access benchmark datasets to support the validation of numerical tools used in aquaculture system design.
Acknowledgment
This project has received funding from the European Union's Horizon Europe research and innovation programme under Grant Agreement No 101093888. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.
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